1. Field of the Invention
This present invention is related to electrically controlled propellants, and in one exemplary embodiment to an improved electrically controlled propellant exhibiting improved performance and safety through resistance to ignition by electrical static discharge at ambient pressures. Methods for using the same are disclosed.
2. Description of the Related Art
There are numerous applications for gas-generating propellant compositions. Often in these applications it is desirable to control the ignition, burn rate, and extinguishment of a propellant by the application of an electrical current. Traditional uses of such gas-generating propellant compositions include rocket propulsion systems, fire suppression systems, oil field services, gas field services, mining, torpedoes, safety air bag systems, and other uses where quickly expanding gas is needed. In many of these instances, an electrically controlled propellant may allow the duration and burn rate of the propellant to be precisely controlled.
In some applications (particularly space and weapon systems), a smokeless or otherwise low signature propellant is desired, in which case a nitramine oxidizer is substituted for ammonium perchlorate. In other applications, high burn rate composites are required, in which case nitramines (RDX, HMX) in combination with nitroglycerin or nitrocellulose are used. These propellants are almost always class 1.1 explosives requiring added safety precautions in the production, shipping and storage of the propellants. In addition, high specific impulse (Isp) propellants are usually formed with ammonium perchlorate composites containing aluminum. These composites generate smoke both from the aluminum combustion and the hydrochloric acid generated interacting with moisture. Finally all of the current propellants have shown to be spark sensitive with many accidents occurring from a stray static charge causing ignition of the propellants during manufacturing (core pulling), storage (spark discharges of motor cases) and ignition and explosion in fires.
U.S. Pat. No. 5,734,124 to Bruenner, et al., describes the formation of liquid nitrate eutectic compositions for solid solution or emulsion propellants wherein inorganic nitrate oxidizers are combined in eutectic compositions that place the oxidizers in liquid form at ambient temperatures. These liquid combinations are then used in the preparation of a wide variety of energetic formulations, notably solution and emulsion propellants. The main component of these eutectic compositions is hydroxylamine nitrate. This same oxidizer is utilized in the propellants developed and described in this patent application. While many benefits of liquid propellants are disclosed in Bruenner, et al., no specific examples of solid solution or emulsion propellants are disclosed.
In contrast to conventional liquid propellants, conventional solid propellants combusted with electric power traditionally require high voltage (in the range of kilovolts) pulse discharges resulting in ablation of the propellant surface to produce ionizing gas species that is then accelerated by an electromagnetic field. Propellants such as these suffer from two serious drawbacks. First, conventional energetic solid propellants will not extinguish after the cessation of electrical current, thereby reducing the precision of control. Second, non-energetic solid propellants provide none of their own thrust since the major portion of the thrust is generated by acceleration of the gas generation ions formed from the electrical energy source. In certain instances it would be beneficial to directly generate thrust from the gas generated by the chemical combustion of the propellant. To date neither a liquid, solid or gas phase propellant exists that can provide a dual purpose propulsion system providing chemical thrust for more rapid movement and threat avoidance combined with the potential for low thrust high specific impulse thrust.
U.S. Pat. No. 5,847,315 (Katzakian) demonstrates that a solid propellant developed as a gas generator for air bag inflators prepared with ammonium nitrate eutectic oxidizers and polyvinylamine nitrate polymer also performed as an electrically controllable extinguishable solid propellant (ECESP). This propellant is non-conductive, has a high electrical resistance and can only be readily ignited by the application of AC signals with either short electrical pathways or with a conductive coating applied to the bore surface of a large grain to reduce the power requirements for ignition. Rapid ignition was achieved only by applying both electrical current and high pressure to the composition.
Newer electrically controlled propellants have been developed and are described in U.S. patent application Ser. Nos. 10/136,786, 10/423,072, and 11/787,001 to Katzakian and others. The more recent '786, '072, and '001 applications disclose propellants demonstrating high conductivity, and are referred to as High Performance Electric Propellants (“HIPEP”). The electrically controlled propellant in the '786 and the '072 patent applications comprise an ionomeric oxidizer binder, an oxidizer mix including at least one oxidizer salt and at least one eutectic material that maintains the mix in a liquid form at the processing temperature and a mobile phase which may include at least one polar protic high boiling organic liquid.
The electrically controlled propellant disclosed in the '786 patent application has drawbacks of its own. Under certain circumstances the propellant can melt or soften during combustion, thereby decreasing its effectiveness and potentially undermining the ability of the propellant's use in situations where the propellant must be repeatedly ignited and extinguished. Additionally, the fluid phase has sufficient volatility to slowly evaporate from the surface of the propellant, making its application not suitable for use in the vacuum of space.
The '001 patent application discloses a still further improved propellant with the desirable characteristics that it be processable and curable at or near room temperature, that it have an electrical conductivity at its combustion surface that is significantly higher than that of the body of the propellant, and that it has a low energy threshold for ignition of the propellant and for maintaining of combustion, while still retaining extinguishment properties. In addition, it is highly electrically stable, conductive over a wide temperature range, and exhibits improved resistance to liquefaction during combustion. As a downside, sustained combustion at pressures less than 200 psi without the application of continuous electrical power input is not achievable using any of the '786, '072, and '001 references. Further, burn rates at pressures above 200 psi (at which the propellants would sustain combustion) is lower than conventional energetic composite solid propellants.
U.S. Pat. No. 5,837,931 to Bruenner et al. discloses a propellant that is liquid at room temperature, is useful as a liquid oxidizer, and that forms a solid solution or emulsion type solid propellant made of ammonium nitrate, hydrazinium nitrate, hydroxylammonium nitrate and/or lithium nitrate, including eutectics. These propellants, which contain a metal fuel, a hydrocarbon polymer and the liquid oxidizer, form a gel structure that supports the metal fuel. Bruenner et al. does not suggest liquid propellants that do not require the formation of solid solutions or eutectics.
U.S. Pat. No. 5,451,277 to Katzakian, et al., discloses a method of preparing a solid energetic composition of coated particles and liquid oxidizers. The energetic composition disclosed therein consists of aluminum powder particles coated with the polymer polyvinyl alcohol. Hydroxylammonium nitrate (HAN) is listed as a suitable liquid oxidizer. The particles disclosed therein are described to form porous solid grains for infusion with liquid oxidizer to thereby form a solid propellant grain, ignitable using conventional pyrotechnic igniters.
Extinguishment control and self-sustaining capabilities are described in a conventional double base or AP composite type propellant in U.S. Pat. No. 7,281,367 to Rohrbaugh et al. Here, extinguishment of a solid fuel grain is achieved by fully opening all valves in communication with the chamber pressure vessel resulting in rapid depressurization and extinguishment of the propellant. Reignition of the rocket motor is effected upon repressurization of the pressure vessel through closing at least some of the valves. In this application, multiple igniters are often necessary to either shorten reignition time or to effect reignition of the propellant.
The capability to sustain combustion at pressures lower than 200 psi without the input of electrical power is therefore important, and lacking in the art. Because many of the applications (particularly space vehicles) characteristically have a limited power supply, a highly efficient composition is also desirable.